The broad gain bandwidth of conventional fiber-laser systems allows for operation over a wide range of wavelengths, or even tunable operation. For the simplest fiber laser system with cavity mirrors having reflectivity across a broad range of wavelengths, the output wavelength can be very broad and can vary with pump power, fiber length, and/or other parameters. The power that can be generated from fiber lasers and fiber-laser amplifiers can often be limited by nonlinear optical effects in the gain and/or delivery fibers used in the system.
It is desirable to produce high peak and average powers from fiber lasers and amplifiers. Stimulated Brillouin scattering (SBS) and other nonlinear effects such as self-phase modulation (SPM), four-wave mixing (FWM), and stimulated Raman scattering (SRS) are the main effects limiting the output power and pulse energy of a fiber amplifier or laser. To suppress these effects in a fiber amplifier/laser, it is desirable to use a rare-earth (RE) doped fiber with a large core. The large core provides two benefits: Spreading the light over a larger core decreases the intensity driving the nonlinear processes, and increasing the core/cladding diameter ratio increases pump absorption, enabling the shortening of the fiber to further reduce nonlinearities. When good beam quality is required, however, increasing the core diameter of the fiber requires that the fiber numerical aperture (NA) be decreased, in order that higher-order modes cannot propagate in the fiber. Using relatively large-core, low-NA fibers with mode-filtering techniques has been demonstrated to achieve good beam quality, but there are practical disadvantages to the use of such fibers. Fibers with very low values of NA exhibit large bending losses, even for relatively large-radius bends. With fibers having the lowest NA, the fiber must be kept quite straight, otherwise the optical amplifier and/or laser has very low efficiency as the bending loss becomes too high. Since a typical laser oscillator or amplifier might require on the order of a meter or more of gain fiber, the inability to coil the fiber has precluded compact packaging of the fiber-laser system.
Stimulated Brillouin Scattering (SBS) is a well-known phenomenon that can lead to power limitations or even the destruction of a high-power fiber-laser system due to sporadic or unstable feedback, self-lasing, pulse compression and/or signal amplification.
Even when a fiber amplifier or fiber laser is designed to compensate for the above effects, there will be a limit on the maximum power that can be obtained from a single fiber when scaling to larger fiber sizes and/or lengths, pump powers, and the like.
U.S. Pat. No. 6,157,763 issued Dec. 5, 2000 to Grubb et al., titled “Double-clad optical fiber with improved inner cladding geometry” and is incorporated herein by reference. This patent described a double-clad optical fiber having an inner cladding with a cross-sectional shape that is non-circular, but that maintains a good end-coupling profile. The introduction of different surface variations into the cross-sectional shape of the inner cladding results in reflective patterns through the cladding that must relatively quickly intersect the core, and such a design is shown in U.S. Pat. No. 4,815,079 to Snitzer et al., which is also incorporated herein by reference. U.S. Pat. No. 5,864,645 to Zellmer, et al. issued Jan. 26, 1999 titled “Double-core light-conducting fiber, process for producing the same, double-core fiber laser, and double-core fiber amplifier”, which is also incorporated herein by reference. The cross-sectional cladding shapes and other aspects described in these patents can be used in some embodiments of the present invention.
It is desirable to prevent or reduce the output of pump-wavelength power from a fiber amplifier or laser. There is a need for improved laser systems, particularly fiber lasers and fiber optical amplifiers having reduced pump-wavelength output power.